The term “heat storage” denotes storing of heat in a substance, and substances that are used for heat storage are referred to as heat storage materials. By heat storage, the temperature of a heat storage material itself, the temperature of the interior of the space in which the heat storage material is disposed, or the like can be maintained substantially constant. For instance, by using heat storage (technologies), solar energy or waste heat can be stored in a substance as heat, and this heat can be used for heating. Furthermore, ice can be generated during the nighttime when power consumption is low, and the produced ice (and heat of fusion of ice) can be used for cooling during the daytime. Thus, through storage of heat, various kinds of energy can be converted into heat and stored, and then re-used. Heat storage technologies therefore play a part in energy conservation being currently advocated. Accordingly, heat storage technologies urgently require further developments.
Heat storage mechanisms can be divided into sensible heat storage and latent heat storage. Sensible heat storage exploits the large specific heat of certain substances. For instance, hot water bottles rely on the large specific heat of water. Latent heat storage exploits the enthalpy of phase transitions. For instance, cooling of drinks using ice water relies on the heat of fusion (enthalpy of fusion) of ice.
In latent heat storage, the enthalpy of a phase transition in a substance is resorted to; as a result, the temperature of the substance can be kept substantially constant, and heat can be added to the substance, and taken from the substance, at a substantially constant temperature of the substance (in sensible heat storage, although temperature changes of the substance with respect to the outside world temperature are small, the temperature of the substance does change gradually nevertheless). Accordingly, technical developments are currently focused on latent heat storage.
Materials for latent heat storage having been developed so far include inorganic salt hydrates, organic materials, molten salts and the like. All the foregoing are heat storage materials that rely on the large enthalpy of solid-liquid phase transitions.
Although large enthalpy changes due to solid-liquid phase transitions are certainly an important factor in heat storage materials, other characteristics, aside from large enthalpy changes, are likewise required from heat storage materials. For instance, it is important that the temperature in the surface of the heat storage material be kept substantially constant over long periods of time; accordingly, heat storage materials are required to exhibit high thermal conductivity. In substances of low thermal conductivity, a large temperature difference arises between the temperature of the surface and the temperature of the interior, and thus the temperature of the surface cannot be kept substantially constant (paraffin, which is an organic material, has low thermal conductivity). When relying on solid-liquid phase transitions in substances that exhibit large volume changes (expansion/contraction) elicited by phase transitions liquid leakage maybe caused. Accordingly, changes in volume derived from phase transitions have to be accordingly small (i.e. in cases where the volume changes are large, a container that withstands the volume changes must be selected as the container of the heat storage material). Further, the heat storage effect is impaired when phase separation or decomposition occurs during phase transitions (in the worst case, the material can no longer be used as a heat storage material). Accordingly, such material s are required not to undergo phase separation or decomposition during phase transitions.